Achieving real-time gas analysis with chemical ionisation
Proton transfer reaction mass spectrometry (PTR-MS) is a portable, field-deployable chemical ionisation technology that is used to detect a wide range of gases with high time resolution and sensitivity at low concentrations. It ionises compounds with proton affinities greater than water, avoiding interference from major air components like nitrogen and oxygen, and enabling detection without sample pretreatment. PTR-MS achieves minimal fragmentation, allowing clear identification in complex mixtures without the extensive fragmentation typically associated with electron impact ionisation, eliminating the need for additional separation steps. Recent advances in PTR-MS have transitioned from quadrupole to time-of-flight mass spectrometers (TOF-MS), further enhancing sensitivity and resolving power to enable more accurate identification of an even broader range of compounds.
PTR-MS is especially useful in time-sensitive environments, providing real-time measurements for:
- Manufacturing – for cleanroom air monitoring and quality control
- Medical diagnostics – for non-invasive breath analysis to assess metabolic health and disease
- Air quality monitoring – for analysing volatile organic compounds (VOCs) from industrial and natural sources
- Foods and flavours – for assessing aroma profiles in product development and quality improvement
Calibration gas reference materials to ensure accurate PTR-MS measurements
PTR-MS detects numerous VOCs, making maintaining accurate calibrations for every detectable compound impractical. Accurate quantification is feasible if the mass-dependent ion transmission rate is well known.1 This needs to be empirically determined to account for deviations from theory resulting from the use of advanced ion optics and poor tuning or aging of the ion detection optics. The current lack of accurate PTR-MS calibration gas reference materials covering a large enough mass range poses challenges to data confidence, as it complicates cross-comparability between different studies and monitoring projects. This underscores the need for a traceable, stable gas reference material to support consistent, comparable and accurate measurements across PTR-MS applications.
NPL is the highest point of traceability, and provides a unique 20-component certified gas reference mixture with a mass range of 32-671 gmol-1 for accurate calibration of the transmission function of PTR-MS instruments – the only reference material of its kind on the market. The composition of this standard includes the following compounds at nominally 1 μmol mol-1 within a nitrogen balance gas, and the uncertainties for each compound are in the range of 3-10 per cent.
| benzene |
m-xylene |
butan-2-one (MEK) |
| 1,2,4-trichlorobenzene |
dimethyl sulfide (DMS) |
acetonitrile |
| 1,2,4-trifluorobenzene |
3-carene |
3-buten-2-one (MVK) |
| 1,2,4-trimethylbenzene |
isoprene |
perfluorotributylamine (PFTBA) |
| hexamethylcyclotrisiloxane (D3-siloxane) |
methanol |
toluene |
| octamethylcyclotetrasiloxane (D4-siloxane) |
acetone |
ethanol |
| decamethylcyclopentasiloxane (D5-siloxane) |
acetaldehyde |
|
Note: Common used names are shown here but the preferred IUPAC names where appropriate are: propan-2-one (acetone), (methylsulfanyl) methane (dimethyl sulfide), 2-methylbuta-1,3-diene (isoprene), but-3-en-2-one (methyl vinyl ketone), butan-2-one (methyl ethyl ketone), 1,3-xylene (m-xylene), 3,7,7-trimethylbicyclo[4.1.0]hept-3-ene (3-carene).
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